   <h1>{{ book.title }}</h1>
   <p>作者：{{ book.author }}</p>
   <p>{{ book.description }}</p>
   <a href="{% url 'index' %}">返回首页</a>
   {% if user.is_authenticated %}
  {% if not book.borrowed_by %}
    <form method="post">
      {% csrf_token %}
      <button type="submit" name="borrow">订阅/借阅</button>
    </form>
  {% elif book.borrowed_by == user %}
    <p>你已订阅（借阅）此书。</p>
  {% else %}
    <p>已被其他人借阅。</p>
  {% endif %}
{% else %}
  <a href="{% url 'login' %}?next={{ request.path }}">登录后可订阅/借阅</a>
{% endif %}
